We put a BRAND NEW 9volt in (also tried with fully charged 9.6 volt)
When he pulls the trigger it doesn't always shoot. We can here the "click" of the solenoid every time. But it seems like it doesn't have enough power to overcome the tension put on by the striker. The problem gets worse the more we increased the velocity.(which increased pressure on the striker and then also on the spring-loaded latch thingy that catches the striker after each shot.)

so it is the same problem you would expect with a low since it seems like the solenoid doesn't have enough power to overcome that force put on the spring-loaded latch thingy by the striker.

Is there maybe a problem with the board, or more likely the solenoid it self that it isn't giving enough power??

To add to what TheDarkShadow said, check for signs of bulging on the capacitor. If it is indeed bulging/blow, Radio Shack has replacements. Just bring in the old one so you know what you're looking for.